Gold recovery process

Abstract

A process for treating refractory ores combines pressure oxidation treatment of the ore with cyanidation and carbon-in-pulp recovery. Pressure oxidation is carried out under acidic conditions, at elevated temperatures and pressure. Before cyanidation, the oxidized slurry is subjected to a multiple stage washing operation to remove excess acid and heavy metals generated during the pressure oxidation. Such heavy metal removal lowers the subsequent cyanide usage and makes the process more economical. Cyanidation is carried out in a conventional manner, and it has been found that carbon-in-pulp recovery leads to greatly enhanced recovery of gold when compared to other conventional methods, such as zinc precipitation.

What is claimed is: 
     
       1. A process for recovering gold from an aqueous slurry formed from a refractory ore where said gold is subject to sulfide locking and adsorption on carbonaceous matter, said process comprising: oxidizing the aqueous slurry in an autoclave under acidic conditions at an elevated temperature and pressure whereby elemental gold is released from the sulfide locking and carbonaceous adsorption and heavy metals present in the ore are solubilized;   separating the solubilized heavy metals from the oxidized aqueous slurry to form an acid liquid fraction containing the majority of the heavy metals and a solids fraction containing the gold ore;   recycling a portion of the acid liquid fraction to the autoclave to provide a source of acid for the oxidation;   disposing of the remaining portion of the liquid fraction to remove excess heavy metals and acid from the process;   dissolving the elemental gold in the solids fraction by exposure to cyanide to form a soluble gold-cyanide complex; and   recovering elemental gold from the gold-cyanide complex.   
     
     
       2. A process as in claim 1, wherein the slurry is oxidized by introducing oxygen to the slurry, heating the slurry to a temperature in the range from about 140° to 220° C.; maintaining a pressure of at least 140 psig, and maintaining a final acidity of at least 5 g/1 H 2  SO 4 . 
     
     
       3. A process as in claim 1, wherein the gold-cyanide complex is recovered by adsorption on activated carbon. 
     
     
       4. A process as in claim 1, wherein the solubilized heavy metals are separated from the oxidized aqueous slurry by washing the slurry and physically separating the washed slurry from excess liquid, whereby the heavy metals are removed with the excess liquid. 
     
     
       5. A process as in claim 4, wherein the slurry is washed and separated at least twice to separate the heavy metals. 
     
     
       6. A process as in claim 5, wherein the slurry is washed by dilution with water and is separated in a clarifier. 
     
     
       7. A process as in claim 6, wherein a flocculant is added to the slurry prior to separation in the clarifier. 
     
     
       8. A process as in claim 1, wherein the elemental gold is dissolved by exposure to a cyanide salt under alkaline conditions with the addition of oxygen. 
     
     
       9. A process as in claim 3, wherein the gold-cyanide complex is adsorbed on the activated carbon in an agitated tank with air addition. 
     
     
       10. A process as in claim 9, wherein there are at least two agitated tanks in series. 
     
     
       11. A process as in claim 3, wherein the elemental gold is recovered from the activated carbon by washing of the carbon with an aqueous caustic cyanide solution, followed by electrowinning. 
     
     
       12. A process for recovering gold from a refractory ore where said gold is subject to sulfide locking and adsorption on carbonaceous matter, said process comprising: comminuting the ore to a preselected size;   forming an acidified aqueous slurry by combining said comminuted ore with an acid and heavy metal recycle stream;   separating the acidified aqueous slurry into an excess acid stream and a residue stream, where said excess acid stream contains solubilized heavy metals and said residue stream contains the gold;   separating the heavy metals from the excess acid stream and neutralizing the acid in the excess acid stream to form a recycle water stream;   oxidizing the residue stream from the acidified aqueous slurry under acidic conditions at an elevated temperature and pressure whereby elemental gold is released from the sulfide locking and carbonaceous adsorption and heavy metals present in the ore are solubilized;   separating the oxidized slurry into a liquid fraction substantially free from solids and a residue fraction containing substantially all of the solids, which liquid fraction serves as the source of the acid and heavy metal recycle stream, whereby the gold is contained in the residue fraction and the majority of heavy metals are in the liquid fraction;   dissolving the elemental gold in the residue fraction of the aqueous slurry by exposure to cyanide to form a soluble gold-cyanide complex;   adsorbing the gold-cyanide salt on activated carbon; and   recovering elemental gold from the activated carbon.   
     
     
       13. A process as in claim 12, wherein the slurry is oxidized by introducing an oxidizing agent to the slurry, heating the slurry to a temperature in the range from about 140° to 220° C.; maintaining a pressure of at least 140 psig, and maintaining a final acidity of at least 5 g/1 H 2  SO 4 . 
     
     
       14. A process as in claim 13, wherein the oxidizing agent is selected from the group consisting of oxygen, ozone, hydrogen peroxide, and sodium hypochlorite. 
     
     
       15. A process as in claim 12, wherein the liquid fraction and residue fraction are separated in a washing circuit comprising a plurality of clarifiers in series where the recycle water stream is used to wash the fractions. 
     
     
       16. A process as in claim 15, wherein the oxidized slurry is diluted with water from the recycle stream prior to introduction to the first clarifier. 
     
     
       17. A process as in claim 16, wherein the oxidized slurry is diluted approximately 3:1 by weight with water from the recycle stream at each stage of clarification. 
     
     
       18. A process as in claim 17, wherein a flocculant is included in the recycle dilution stream. 
     
     
       19. A process as in claim 12, wherein the elemental gold is dissolved by exposure to a cyanide salt under alkaline conditions with the addition of oxygen. 
     
     
       20. A process as in claim 12, wherein the gold-cyanide complex is adsorbed on the activated carbon in an agitated tank with air addition. 
     
     
       21. A process as in claim 20, wherein there are at least two agitated tanks in series. 
     
     
       22. A process as in claim 12, wherein the elemental gold is recovered from the activated carbon by washing of the carbon with an aqueous caustic cyanide solution, followed by electrowinning.